1. Field of the Invention
This invention relates generally to steering systems for tracked vehicles and more particularly to a superposition steering system having a steering transmission which integrates infinitely variable and mechanical input drives.
2. History
Steering systems for tracked vehicles such as military tanks have featured an infinitely variable hydrostatic steering unit comprising variable speed pumps which drove hydraulic motors. The hydraulic motors, in turn, engaged a zero or neutral shaft which was interconnected to a pair of summation planetary gears each of which drove one of the vehicle tracks or chains through an associated sprocket wheel.
The summation planetary gears were primarily driven by a propulsion drive and the engagement with the zero shaft superimposed steering control over the sprocket wheels. Since the hydrostatic units were continuously variable, the steering control superimposed upon the propulsion drive by the zero shaft was infinitely variable. Thus, the vehicle negotiated a curve which was proportionate to the steering angle or degree of steering wheel rotation imposed by the vehicle operator whose steering wheel was linked to the variable speed pump.
Superposition steering systems of the foregoing type with infinitely variable steering drives have been typically illustrated in U.S. Pat. No. 4,327,603 dated May 4, 1982, and U.S. Pat. No. 4,497,218 dated Feb. 5, 1985, both issued to the assignee of the present invention.
The advent of heavier tracked vehicles driven by larger engines resulted in the necessity to provide larger, heavier and more costly hydrostatic drive units capable of generating the increased power necessary to effectuate superimposed steering control. The need to increase steering efficiency so as to reduce the requirements for unduly large hydrostatic units resulted in the development of a combined mechanical and infinitely variable hydrostatic steering drive system as exemplified in U.S. Pat. No. 3,938,604 issued Feb. 17, 1976 to the assignee of the present invention.
Such system included a mechanical steering input drive powered directly from the vehicle engine and an infinitely variable hydrostatic input drive, whose hydrostatic pump was driven from the vehicle engine. The two steering inputs were integrated in a controlled sequence at a mechanical steering transmission. The output of the mechanical steering transmission drove a neutral shaft for superimposed steering control of sprocket wheels which drove vehicle tracks.
The system disclosed in U.S. Pat. No. 3,938,604 resulted in a significant reduction in the power requirements of hydrostatic units to 1/3 of the total power required for steering control. The remaining power for steering control was provided through the mechanical input drive.
There remained a need, however, for still further reductions in the power requirements of variable drives and for further improvements in steering efficiency at relatively large turning radii. Increases in steering efficiency at large turning radii were also desired to obtain precise positioning of large guns mounted to turretless tracked vehicles.
In addition, faster response and high maneuverability mandated by battlefield conditions remained a goal for tracked vehicle steering systems.